Ok thanks I'll try and take some picturesAlright. Look at the back of the case and count the expansion slots:
2 or 3 is Mini-ITX. 4 or 5 is Micro-ATX. 6 or more is ATX or EATX.
See what rear fan size it has. 80mm or 120mm or 140mm. That'll give an indication of the size of CPU cooler you can use.
Find out which power supply it is (brand and model) and which monitor (resolution and refresh rate).
If possible, take pics of the case and power supply.